Proximity Induced Magnetic Anisotropy and Trefoil Fermiology in Monolayer FeCl₂/Bi(111)

Interfaces between magnetic and non-magnetic materials play a crucial role in various magnetic heterostructures. The emergence of 2D van der Waals (vdW) magnets has introduced new opportunities for exploring proximity effects in vdW heterostructures. While the influence of magnetic layers on nearby non-magnetic materials has been widely studied, it remains unclear whether non-magnetic substrates can similarly modulate the intrinsic magnetic properties of 2D magnets, particularly their magnetic anisotropy. In this work, by analyzing X-ray magnetic circular dichroism spectra of an epitaxially grown FeCl₂ monolayer on a Bi(111) surface, a reorientation of magnetic anisotropy is observed - from its natural out-of-plane to a predominantly in-plane alignment. This effect vanishes in bilayer FeCl₂/Bi(111), where the magnetic anisotropy reverts to its intrinsic out-of-plane orientation, consistent with the layered antiferromagnetic order of bulk FeCl₂. Angle-resolved photoelectron spectroscopy reveals the presence of metallic interface states derived from the Bi surface states, accompanied by charge transfer and emergence of a moiré potential that gives rise to a distinctive trefoil-shaped Fermi surface. These results demonstrate that non-magnetic substrates can exert strong proximity influence on the magnetic and electronic behavior of 2D vdW magnets, offering new strategies for engineering magnetic anisotropy and electronic structure in spintronic heterostructures.